The grating measurement system as a typical displacement sensor is widely applied to many mechanical and electrical equipment. The measuring principle of the grating measurement system is mainly based on the principle of Moire fringe and the principle of diffraction and interference. The grating measurement system based on the principle of Moire fringe is a mature displacement sensor having become the first choice of displacement measurement for many mechanical and electrical equipment due to its numerous advantages such as long ranging, low cost, easy alignment, etc., but its accuracy is usually at the micron scale, and it is commonly seen in general industrial applications.
In semiconductor manufacturing equipment, a photoetching machine is the key equipment in the manufacture of semiconductor chips. An ultra-precise workpiece stage is the core subsystem of the photoetching machine, and is used for carrying mask plates and silicon wafers to implement high-speed ultra-precision stepping and scanning movement. The ultra-precise workpiece stage becomes the most typical system in ultra-precise movement systems due to its movement characteristics, such as high-speed, high acceleration, large stroke, ultra-precision, and multi-DOF, etc. In order to achieve the above movements, the ultra-precision workpiece stage usually adopts a dual-frequency laser interferometer measurement system to measure multi-DOF displacements of the ultra-precision workpiece stage. However, as the continuous improvement of such movement indicators as measurement accuracy, measurement distance, measurement speed, etc., the dual-frequency laser interferometer is difficult to meet higher measurement demands due to a series of problems, such as environmental sensitivity, difficulties in improving measurement speed, large space occupation, expensive price, poor in measuring the dynamic characteristics of the target work stage, etc.
With respect to the above-mentioned problems, major companies and research institutions in the field of ultra-precision measurement all around the world launched a series of researches, concentrating mainly on grating measurement system based on the principle of diffraction and interference, and the research results are disclosed in many patents and papers. U.S. Pat. No. 7,102,729 B2 (publication date: Aug. 4, 2005), U.S. Pat. No. 7,483,120 B2 (publication date: Nov. 15, 2007), U.S. Pat. No. 7,940,392 B2 (publication date: Dec. 24, 2009), and Publication No. US2010/0321665 A1 (publication date: Dec. 23, 2010) disclose a plane grating measurement system applied to a ultra-precision workpiece stage of a photoetching machine and its arrangement scheme, where the measurement system mainly uses an one-dimensional or two-dimensional plane grating coordinating with a reading head so as to measure large stroke horizontal displacement of the workpiece stage; and adopts a sensor, such as an eddy current sensor or an interferometer, etc., to measure vertical displacement, but the application of a variety of sensors limits the measurement accuracy of the workpiece stage. U.S. Patent Document Publication No. US2011/0255096 A1 (publication date: Oct. 20, 2011) discloses a grating measurement system applied to a ultra-precision workpiece stage of a photoetching machine, where the measurement system also uses an one-dimensional or two-dimensional grating coordinating with a particular reading head so as to implement displacement measurement and can conduct horizontal and vertical displacement measurements simultaneously, but its structure is complicated. U.S. Patent Document Publication No. US2011/0096334 A1 (publication date: Apr. 28, 2011) discloses a heterodyne interferometer, where the interferometer adopts a grating as the target mirror, but the interferometer can only implement one dimensional measurement. Japanese scholar GAOWEI presents a single-frequency two-dimensional grating measurement system, which utilizes the principle of diffraction and interference, in research paper “Design and construction of a two-degree-of-freedom linear encoder for nanometric measurement of stage position and straightness. Precision Engineering 34(2010)145-155”, where the grating measurement system can implement horizontal and vertical displacement measurements simultaneously, but due to the adoption of the single frequency laser light, the measurement signal is susceptible to interference, and accuracy is difficult to be guaranteed. Chinese patent literature Application No. 201210449244.9 (filling date: Nov. 9, 2012) and 201210448734.7 (filling date: Nov. 9, 2012) respectively disclose a heterodyne grating interferometer measurement system, where both the interferometer measurement systems adopt, in their reading head structures, quarter waveplates to change the polarization state of light beam, the optical structure is complex, while the non-ideal properties of the optical element will result in measurement errors.